A good jungler will know precisely how to kill neutral monsters very quickly and effectively (HP + kill order). Having jungler implies having two champions soloing a lane, so both solo champions gain more experience than either of the enemies in a two-champion lane.

If the jungler is experienced enough, he can keep up with the level of laning champions, but it's a job! Don't expect to succeed on the first time. Practice!
Usually (and unfortunately) some champs will have difficulties to do their jobs as jungler without runes. (Since the bonus they provide can be quite significant at level 1) You will have to request help to kill your first monster and make good use of healing potion. (Take care, as they heal over time and not instantly!)

Then it will be up to you to know when to stop jungling (past a certain level it becomes useless) and also when to provide help to other lanes (since you are invisible most of time, the enemy will be always fearing you... but you have to show up sometimes for that).

Now the speed and efficiency of the jungler will change the Experience and Gold they get but average junglers tend to keep up with the players who are in lanes with partners or better while allowing two champions to solo. Other added benefits include less predictable ganking as the jungler can roam anywhere out of sight of the enemy team without the enemy laner calling them "mia" and better control of the dragon.

a very thorough and well explained answer, thank you :)
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Daniel RedfordMar 25 '12 at 6:09

2

You missed the fifth thing - the team averages more XP per minute! You're also taking the XP that would just be sitting in the jungle while your solo top gets all the XP in that lane, instead of splitting 3 lanes of XP 5 ways.
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lilserfMar 25 '12 at 6:10

If you have a jungler, the overall xp and gold gain is going up for your team. The solo laner gets to kill more minions (and perhaps champions) and the jungler earns his xp and money in the jungle. win/win for both of you.

By having a Jungler, your team will have 2 solo lanes, giving you 2 characters that a re higher levels and hopefully more gold than your opponents. In addition, some junglers get really high leveled quickly and can pick up more gold as well. Junglers often gank the enemy for their team, allowing them to recover from unfortunate situations and scaring the enemy so they won't be too aggressive. Also, in the jungle there are buffs that can help your team, such as Dragon that grants gold to your team and Red and Blue buff that can help the champion who kills the monsters that guard it; the jungler may take these alone or give them to allies to help them out as well.

So wait what your saying is depending on who you jungle with you will level faster? Or the champions in the lanes will level faster?
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conjimMar 25 '12 at 6:08

@conjim What he said is correct, heroes that have high AoE damage such as Amumu, Malphite, Rammus and Skarner level faster than heroes like Warwick who auto-attack one target at a time.
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Daniel RedfordMar 25 '12 at 6:11